In the situation you describe - where you want maximum detail in your subject but it covers only a small part of the frame - it's all about the number of pixels.
Your D300s has 4,288 pixels along the long side of its sensor.
The D3100 has 4,608 - an increase of 7.5% which is pretty small. The D800 in DX crop mode would give you 4,800 pixels, still only 12% more than the D300s. A D7000 is a little more again, 4,928 or 15% greater than the D300s. In practice, don't think you would notice much improvement from any of those.
On the other hand, a 24MP DX camera like the postulated "D400" would give you 6,000 pixels, which is a quite noticeable 40% increase over the D300s. Maybe that would be worth waiting for...